Organic electroluminescent device including transparent conductive layer and fabricating method thereof

ABSTRACT

A method of fabricating an electrode for an organic electroluminescent device includes forming a transparent conductive layer on a substrate, doping the transparent conductive layer with impurities, and annealing the doped transparent conductive layer.

This application is a Divisional of U.S. patent application Ser. No.10/330,298, filed Dec. 30, 2002 now U.S. Pat. No. 6,927,535 and claimsthe benefit of the Korean Patent Application No. P2002-14170 filed inKorea on Mar. 15, 2002, both of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electroluminescent device,and more particularly, to an organic electroluminescent device includinga transparent conductive layer and a fabricating method thereof.

2. Discussion of the Related Art

In general, liquid crystal display (LCD) devices are commonly used forflat panel displays (FPDs) because they are lightweight and consumerelatively low amounts of power. However, LCD devices are notlight-emitting displays. As such, LCDs have several disadvantagesincluding dim displays, poor contrast ratios, narrow viewing angles andsmall display sizes. Accordingly, new FPDs, such as organicelectroluminescent (EL) devices, have been developed to solve theseproblems. Organic EL devices are light-emitting displays that possess awider viewing angle and a better contrast ratio than LCD devices.Furthermore, since no backlight is required for an organic EL device,organic EL devices generally are both lighter and thinner than LCDdevices, and consume less power. Organic EL devices may be driven with alow direct current (DC) voltage that permits a faster response speedthan LCD devices. Moreover, since organic EL devices are solid-phasedevices, unlike LCD devices, they can better withstand external impactsand possess a greater operational temperature range. In addition,organic EL devices may be manufactured more cheaply than LCD devices orplasma display devices (PDPs) because organic EL devices require onlydeposition and encapsulation apparatus.

A passive matrix design that does not use additional thin filmtransistors (TFTs) may be used for organic EL devices. However, passivematrix organic EL devices have limited display resolution, relativelyhigh power consumption, and a relatively short expected life span. Thus,active matrix organic EL devices have been developed as next-generationdisplay devices that provide high resolution over a large display area.In passive matrix organic EL devices, a scan line and a signal linecross each other to provide a switching element for a sub-pixel. Incontrast, a TFT, disposed at each sub-pixel, is used as a switchingelement in active matrix organic EL devices. The TFT is used to turneach sub-pixel ON or OFF. Specifically, a first electrode, which isconnected to the TFT, is turned ON or OFF by the sub-pixel, and a secondelectrode, which faces the first electrode, functions as a commonelectrode.

FIG. 1 is an energy band diagram of an organic electroluminescent deviceaccording to the related art. In FIG. 1, the organic electroluminescentdevice includes an anode 1 and a cathode 7 that are separated from eachother, a hole injection layer 2, a hole transporting layer 3, anemission layer 4, an electron transporting layer 5, and an electroninjection layer 6 interposed between the anode 1 and the cathode 7. Thehole injection layer 2 and the electron injection layer 6 contact theanode 1 and the cathode 7, respectively. A hole that is on the anode 1passes through the hole injection layer 2 and the hole transportinglayer 3 and is injected into the emission layer 4. An electron that ison the cathode 7 passes through the electron injection layer 6 and theelectron transporting layer 5 and is injected into the emission layer 4.The hole and the electron injected into the emission layer 4 form anexciton 8, and light is emitted by the formation of the exciton 8. Sincemobility of the hole and mobility of the electron are substantiallydifferent in an organic material, the hole and the electron areeffectively transported to the emission layer by using the holetransporting layer and the electron transporting layer. Thus, themulti-layer organic electroluminescent device has a high emissionefficiency due to a balance of densities of holes and electrons in theemission layer.

The anode 1 may be made of a transparent conductive material such asindium-tin-oxide (ITO) and indium-zinc-oxide (IZO). The cathode 7 may bemade of a chemically stable material whose work function is lower thanthat of the anode 1. As the work function of the cathode is lowered, alower driving voltage is required. Moreover, a lower work functionresults in improved brightness and current density. The cathode may bemade of aluminum (Al), calcium (Ca), lithium:aluminum (Li:Al), ormagnesium:silver (Mg:Ag).

In FIG. 1, since an anode 1 made of ITO has a work function betweenabout 4.7 eV and about 4.8 eV, and the hole injection layer 2 has a workfunction between about 5.2 eV and about 5.3 eV, an energy band gap “I”exists between the anode 1 and the hole injection layer 2. Accordingly,the injection efficiency of a hole from the anode 1 is reduced due tothe work function difference between the anode 1 and the hole injectionlayer 2. Since an organic electroluminescent device uses carrierinjection, the device performance is reduced by the loss of injectionefficiency of the carrier. Moreover, adhesion between the anode and theorganic thin film is basically poor in the organic electroluminescentdevice. As a result, the anode and the organic thin film may separatedue to differences in their interface voltages and thermal expansioncoefficients at high driving voltages or high temperatures. Accordingly,degradation of the organic electroluminescent device may occur, whichmay result in a reduction in the expected life span of the organicelectroluminescent device.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an organicelectroluminescent device and a fabricating method thereof thatsubstantially obviate one or more of the problems due to limitations anddisadvantages of the related art.

An object of the present invention is to provide an organicelectroluminescent device with improved injection efficiency, expectedlife span and display quality, and a fabricating method thereof.

Another object of the present invention is to provide an organicelectroluminescent device and a fabricating method thereof where ananode of a transparent conductive layer is doped with impurities andannealed.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a method offabricating an electrode for an organic electroluminescent deviceincludes steps of forming a transparent conductive layer on a substrate,doping the transparent conductive layer with impurities, and annealingthe doped transparent conductive layer.

In another aspect, a method of fabricating an organic electroluminescentdevice includes forming a transparent conductive layer on a substrate,doping the transparent conductive layer with impurities, annealing thedoped transparent conductive layer, forming a plurality of organicelectroluminescent layers, to include a first organic electroluminescentlayer and a second electroluminescent layer such that the first organicelectroluminescent layer is coupled to the first electrode, and forminga second electrode coupled to the second organic electroluminescentlayer.

In another aspect, an organic electroluminescent device includes asubstrate, a first electrode on the substrate, a plurality of organicelectroluminescent layers, the plurality of organic electroluminescentlayers include a first organic electroluminescent layer and a secondorganic electroluminescent layer, and the first organicelectroluminescent layer is coupled to the first electrode, and a secondelectrode coupled to the second organic electroluminescent layer.

In another aspect, an electrode of an organic electroluminescent deviceis produced by a process comprising forming a transparent conductivelayer on a substrate, doping the transparent conductive layer withimpurities, and annealing the doped transparent conductive layer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is an energy band diagram of an organic electroluminescent deviceaccording to the related art;

FIGS. 2A and 2B are schematic cross-sectional views showing an exemplaryprocess for fabricating a transparent conductive layer for an organicelectroluminescent device according to the present invention; and

FIG. 3 is an energy band diagram of an exemplary organicelectroluminescent device according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

FIGS. 2A and 2B are schematic cross-sectional views showing an exemplaryprocess for fabricating a transparent conductive layer for an organicelectroluminescent device according to the present invention. In FIG.2A, after a transparent conductive layer 112 is formed on a substrate110, the transparent conductive layer 112 may be doped with impurities.The substrate 110 may be a transparent substrate or an array substratedepending on the method of driving the organic electroluminescentdevice. The transparent conductive layer 112 may includeindium-tin-oxide (ITO) or indium-zinc-oxide (IZO), for example. Theimpurities may be positive type (p-type) compounds containing boron (B)such as B₂H₆ or H₂ (B₂H₆: H₂=20%: 80%), between about 5×10¹⁴ ions/cm²and about 3×10¹⁵ ions/cm², and may be accelerated with an accelerationvoltage of about 30 KV. Alternatively, the impurities may be made ofnegative type (n-type) compounds containing phosphorus (P), such as PH₃.The transparent conductive layer 112 may be about 1400 Å thick, and amaximum concentration depth (R_(p)) of the doped impurities may be about40 Å from a top surface of the transparent conductive layer 112.Preferably, the maximum concentration depth may be about 30% of thethickness of the transparent conductive layer 112. When the transparentconductive layer 112 is doped with impurities, its work function may beincreased by between about 0.04 eV and about 0.14 eV. The resultingincrease of the work function may be measured by using a cyclicvoltamogram (CV), for example. The transparent conductive layer 112 mayfunction as a first electrode of the organic electroluminescent device.A hole injection layer (not shown) made from an organic material maysubsequently be formed on the first electrode by a separate process. Theincrease in the work function of the transparent conductive layer 112caused by doping the transparent conductive layer 112 with impuritiesmay reduce the difference in work functions between the first electrodeand the hole injection layer. As a result, an injection efficiency of acarrier, such as a hole, may be improved.

In FIG. 2B, after the transparent conductive layer 114 is doped with theimpurities, the transparent conductive layer 114 may be annealed. Theprocess of annealing the doped transparent conductive layer 114 mayreduce surface roughness and increase transmittance. The annealingprocess may be performed at a temperature between about 230° C. andabout 400° C. for between about 30 minutes and about 2 hours.Preferably, the doped transparent conductive layer 114 may be annealedat a temperature of about 400° C. for about 30 minutes. Exemplarytransparent conductive layers are compared in Table 1. Each sample usedto produce the results presented in Table 1 had a transparent conductivelayer made of ITO.

TABLE 1 Doping condition (DOSE/ Doping Acceleration Voltage) ThicknessAnnealing Resistivity Transmittance Sample 1 X — 1400 Å O 207.2 Ω · cm94% Sample 2 X — 1400 Å X l95.7 Ω · cm 79% Sample 3 O 3 × 10¹⁵/30 KV1400 Å O 211.5 Ω · cm 95% Sample 4 O 5 × 10¹⁴/30 KV 1400 Å X 495.2 Ω ·cm 80% Sample 5 O 5 × 10¹⁴/30 KV 1400 Å O 156.5 Ω · cm 96%

Samples 1 and 2 were not doped with impurities, and Samples 2 and 4 werenot annealed. In Table 1, the transparent conductive layer of Sample 5has a lower resistivity than the other samples. Moreover, thetransparent conductive layer of Sample 5 has an equivalent transmittanceto that of Sample 1.

FIG. 3 is an energy band diagram of an exemplary organicelectroluminescent device according to the present invention. Anemission layer, an electron transporting layer, and an electroninjection layer have been omitted in FIG. 3 for clarity. The structureof FIG. 1 can be used to incorporate the omitted structures of FIG. 3.In FIG. 3, an organic electroluminescent device 200 includes an anode212, a hole injection layer 214, and a hole transporting layer 216. Theanode 212 may include a transparent conductive layer that has been dopedwith impurities. Accordingly, the difference between the work functionof the anode 212 and the work function of the hole injection layer 214(“II”) is less than the difference between the work function of therelated art anode 1 and the work function of the hole injection layer214 (“I”). Thus, the injection efficiency from the anode 212 to the holeinjection layer 214 is greater than the injection efficiency from therelated art anode 1 to the hole injection layer 214. As a result,characteristics of the organic electroluminescent device 200, such asbrightness, may be improved as a result. Furthermore, the process ofannealing the transparent conductive layer for the anode 212 may resultin reduced surface roughness, increased transmittance, and strengthenedconnectivity between the anode 212 and the hole injection layer 214,thereby increasing the expected life span of the organicelectroluminescent device.

The hole injection layer 214 and the hole transporting layer 216 can bemade of either a high molecular substance or a low molecular substance.If a low molecular substance is used, the hole transporting layer 216may be omitted. The organic electroluminescent device of the presentinvention may be either an active matrix organic electroluminescentdevice or a passive matrix organic electroluminescent device.Furthermore, the transparent conductive layer of the present inventionmay be used as a top emission electrode or a bottom emission electrodedepending upon the direction in which light is to be emitted.

Therefore, by reducing the difference between the work functions at theinterface of an electrode and an organic thin film, the properties of anorganic electroluminescent device are improved. Moreover, by reducingthe surface roughness and increasing the transmittance of the electrodeby an annealing process, display quality is improved and the expectedlife span of the device is increased.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the organicelectroluminescent device and the fabricating method thereof of thepresent invention without departing from the spirit or scope of theinvention. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1. A method of fabricating an electrode for an organicelectroluminescent device, comprising steps of: forming a transparentconductive layer on a substrate; doping the transparent conductive layerwith accelerated impurities; and annealing the doped transparentconductive layer, wherein the electrode is coupled to a hole injectionlayer, wherein the transparent conductive layer includes at least one ofindium-tin-oxide And indium-zinc-oxide, and wherein the hole injectionlayer has a work function between about 5.2 eV and About 5.3 eV, and thetransparent conductive layer has a work function of about 4.84 eV. 2.The method according to claim 1, wherein the transparent conductivelayer is doped with impurities between about 5×10¹⁴ ions/cm² and about3×10¹⁵ ions/cm² at an acceleration voltage of about 30 kV.
 3. The methodaccording to claim 1, wherein the step of annealing the dopedtransparent conductive layer includes annealing the doped transparentconductive layer at a temperature between about 230° C. and about 400°C. for a time period between about 30 minutes to about 2 hours.
 4. Themethod according to claim 1, wherein the step of annealing the dopedtransparent conductive layer includes annealing the doped transparentconductive layer at a temperature of about 400° C. for about 30 minutes.5. The method according to claim 1, wherein the step of doping thetransparent conductive layer includes doping the transparent conductivelayer to a maximum concentration depth equal to about 30% of a thicknessof the transparent conductive layer.
 6. The method according to claim 1,wherein the impurities are positive type (p-type) impurities.
 7. Themethod according to claim 6, wherein the impurities are compoundsincluding boron impurities.
 8. The method according to claim 1, whereinthe impurities are negative type (n-type) impurities.
 9. The methodaccording to claim 8, wherein the impurities are compounds includingphosphorus impurities.
 10. A method of fabricating an organicelectroluminescent device, comprising steps of: forming a transparentconductive layer on a substrate; doping the transparent conductive layerwith accelerated impurities; annealing the transparent conductive layerdoped with the accelerated impurities to form a first electrode; forminga plurality of organic electroluminescent layers to include a firstorganic electroluminescent layer and a second organic electroluminescentlayer such that the first organic electroluminescent layer is coupled tothe first electrode; and forming a second electrode coupled to thesecond organic electroluminescent layer, wherein the transparentconductive layer includes at least one of indium-tin-oxide andindium-zinc-oxide, and wherein the first organic electroluminescentlayer has a work function between about 5.2 eV and about 5.3 eV, and thetransparent conductive layer has a work function of about 4,84 eV. 11.An electrode of an organic electroluminescent device produced by aprocess comprising: forming a transparent conductive layer on asubstrate; doping the transparent conductive layer with acceleratedimpurities; and annealing the doped transparent conductive layer,wherein the electrode is coupled to a hole injection layer, wherein thetransparent conductive layer includes at least one of indium-tin-oxideand indium-zinc-oxide, and wherein the hole injection layer has a workfunction between about 5.2 eV and about 5.3 eV, and the transparentconductive layer has a work function of about 4.84 eV.